No research has gone into this but I think we can all agree that the 5k is the most raced distance in the US. I don't know why but it's always bothered me that the distance markers are typically in miles. Any race directors out there care to comment. Why won't you mark the course in Kilometers if that is the advertised distance you hosting. If you feel the need to mark in miles advertise a 3.10686m race.
Because Americans are dumb and don't know what K is
This is why my watch is set to kilometers. Not totally accurate but the splits are pretty darn close.
Almost all Americans train using miles.
Almost all Americans know pace themselves using using minutes/mile, kilometer splits would be useless to the vast majority of participants in a race.
The real question is why are racing 5, 8, and 10k instead of 3, 5, and 6 miles.